The use of battery powered smoke detectors is becoming widely accepted because they can be readily installed anywhere in a building by a person other than a qualified electrician.
Most smoke detectors comprise at least one ionization chamber having a radio-active source for ionizing molecules of air in the chamber and a pair of electrodes for establishing an electric field in the chamber region between them.
A problem associated with an ionization chamber connected to a battery is that the smoke detector sensitivity increases substantially and upwards of 30% upon an approximate 20% decrease in battery voltage. This substantial increase in sensitivity can result in the detector setting off an alarm for an insignificant concentration of smoke particles. This is annoying to the consumer and results in loss of confidence in the detector.
Further, there has been a problem in driving electronic multi-vibrator circuitry which activates an indicator means on a regular interval to indicate to the owner that the unit is receiving power. Most known multi-vibrators which are economically feasible for use in smoke detectors, draw far too much power from the battery so that they overlimit the battery life.
The improvements according to this invention provide a smoke detector which has constant sensitivity over the useable life of the battery and which has micro-power astable multi-vibrator circuitry for activating a power "on" indicator on a regular interval.
In most smoke detectors of the type comprising at least one ionization chamber, an increase in voltage drop across the chamber is detected as caused by entry of combustion products. A predetermined concentration of combustion products corresponding to a least hazardous concentration results in an increment of voltage drop across the chamber for a fully charged battery. According to an aspect of the invention a first electronic means is provided to monitor the voltage drop across the ionization chamber. The first electronic means has a predetermined alarm threshold at which it causes an alarm signal corresponding to a voltage drop over the chamber for a fully charged battery at the predetermined concentration of combustion products. As mentioned, as the battery voltage decreases, the sensitivity of the detector increases. This is due to the increment in voltage drop increasing as the battery voltage decreases for the same concentration of combustion products. A second electronic means is provided to adjust upwardly the alarm threshold of the first electronic means as the battery voltage decreases. In so doing the first electronic means causes the alarm at approximately the same concentration of combustion products in the air throughout the useful life of the battery. This therefore achieves the desired constant sensitivity of the smoke detector.
In the instance where a pair of ionization chambers are employed, they are electrically connected in series. One of the chambers is open to atmosphere and the other is impervious to combustion products. The battery applies a voltage across the pair of chambers where a concentration of combustion products in the open chamber causes imbalanced voltage drops across the chambers. The first electronic means is adapted to monitor the voltage level between the pair of chambers. The alarm threshold of the first electronic means corresponds to a detected imbalanced voltage drop across the chambers caused by the predetermined concentration of combustion products. The pair of ionization chambers may be modified where the chamber which is impervious to smoke is replaced by a resistor means of a resistance equivalent to such chamber.
The micro-power multi-vibrator according to this invention draws very little current in providing a pulse on a prolonged interval, such as every 60 seconds, to activate a means to indicate a diagnostic function. For example, the signal may be used to activate a visual means to indicate that the unit is receiving power or activate an alarm on an intermittant basis to indicate that the unit is not functioning properly or that the battery is approaching the end of its useful life. The astable multi-vibrator circuitry comprises first and second inverters, each inverter including a pair of complimentary transistors. The output of the first inverter is coupled through the input of the second inverter. A capacitor couples the output of the second inverter to the input of the first inverter. A first resistor is coupled between the input and the output of the first inverter. A second resistor of substantially less resistance than the first is coupled with a diode in parallel with the first resistor. This provides an unbalanced charge time and discharge time for the compacitor so that the "off" time of the visual indicator is substantially greater than its "on" time. This multi-vibrator circuitry is improved upon by providing means for limiting current drawn during switching of states of the first and second inverters and means for maintaining a voltage on the inverters during their switching of states. The level of voltage provided is above a minimum voltage required to operate the inverters and is below the voltage applied to the inverters by the battery. Such means insures a reliable fast switching or inverter states. This results in very little current draw during multi-vibrator operation. Means is provided to derive a pulse output from the second inverter to activate the indication means.
According to another aspect of the invention in the multi-vibrator circuitry where field effect transistors are used the means for maintaining a voltage on the inverters may be adapted to decrease the voltage applied over the period of switching.